The automotive industry is trying develop brighter and better head lamps for automobiles. The conventional incandescent lamp cannot be modified to increase its brightness, so the automotive industry is turning to halogen lamps to improve the output brightness of car head lamps. Halogen lamps have significantly higher output brightness.
The automotive companies are investigating arc-lamps to improve lamp brightness. Arc-lamps are still experimental. Arc-lamp lifetimes have not been proven to be adequate and the cost of arc lamps are still too high.
Another possible solution to the brightness concerns of incandescent lights or halogen lamps is to use an anti-reflection coating on the head lamp lens. A head lamp 20 is shown in FIG. 1. The head lamp 20 comprises a light bulb 22, which can be incandescent, halogen or arc lamp. A reflector 24 directs the light forward. A head lamp lens 26 protects the light bulb 22 and reflector 24 from bugs and other objects. The head lamp lens 26 is commonly made of polycarbonate (older models were made from glass). The reflection from each surface of the polycarbonate is about 5%. This results in a 10% reduction in the output optical power after passing through the head lamp lens 26.
Anti-reflective coatings use the principle of destructive interference to reduce the reflectance from a surface. A glass pane had an index of refraction (n=1.5) that differs from the index of refraction of the surrounding air (n=1.0). The discontinuity in the index of refraction results in a certain percentage of the impinging light being reflected. By placing a coating on the glass of the appropriate thickness and an index of refraction, it is possible to create destructive interference between the reflection from the air-coating boundary and the coating-glass boundary.
A common anti-reflective coating is shown in FIG. 2. A glass substrate 30 has an index of refraction of about 1.5 and is coated with magnesium fluoride 32. The magnesium fluoride 32 is applied with a thickness 34 equal to 1/4 of the wavelength of the center of the visible spectrum (550 nm). The 1/4 wavelength coating results in a 1/2 wavelength phase difference for the reflected light from the substrate 30 magnesium fluoride boundary. As a result this light is 180 degrees (destructive interference) out of phase with the light reflected from the air- magnesium fluoride boundary. To eliminate any reflection it is necessary for the amplitude of each of the reflected light waves to be equal. The magnesium fluoride has an index of refraction of 1.38 so the thickness 34 is 99.6 nanometers (550 nm* 1/4* 1/1.38). The magnesium fluoride coating reduces the reflectance at each surface to about 1% or 2% through the head lamp lens. However, this still has a 2% reflectance.
Thus there exists a need for an optical coating that further reduces the reflectance from the head lamp lens.